1. Technical Field
This invention relates to novel preceramic silazane polymers, a method of preparing such polymers from silicon amides and polyamines and ceramics prepared from these polysilazanes.
2. Description of Related Art
The development of new and improved preceramic silicon-containing polymers continues to be of significant interest. Of particular interest are silazane polymers because upon pyrolysis these polymers convert predominantly to silicon carbide and/or silicon nitride which are useful in the preparation of oxidation resistant, high temperature materials.
A common method for preparing silazane polymers involves the use of halogenated reactants such as halosilanes. For example, U.S. Pat. No. 3,885,567 discloses shaped articles comprising a homogeneous mixture of silicon carbide and silicon nitride. These articles are made from carbosilazane resins which are derived from the reaction of chlorosilanes and amines.
U.S. Pat. Nos. 4,395,460 and 4,404,153 disclose processes for preparing polysilazane polymers by contacting and reacting disilanes with ammonia and disilazane, respectively.
U.S. Pat. No. 4,543,344 discloses a process for preparing R.sub.3 SiNH-containing hydrosilazane polymers by contacting and reacting trichlorosilane with a disilazane (R.sub.3 Si).sub.2 NH where R is vinyl, hydrogen, phenyl or alkyl radicals containing 1 or 3 carbon atoms.
These processes which involve the use of halogen containing silazanes to prepare polysilazanes require difficult procedures to eliminate the halogen containing by-products created by this reaction.
A process for preparing halogen free polymers has been proposed in U.S. Pat. No. 4,675,424 which describes a method for preparing silazane polymers by reacting in the presence of catalyst an aminosilane with ammonia or other useful amines. The preferred amines are ammonia and monomethylamine. The patent discloses higher equivalent weight amines such as ethylenediamine, propylamine, allylamine and aniline.
There is still a need to develop preceramic polymers that are free of catalytic impurities and have improved properties which make them suitable for lamination uses and allow the polymers to retain much of their strength during pyrolysis.